U.S. Pat. No. 5,014,926 discloses a seat belt retractor having a web-sensitive locking mechanism. The seat belt retractor disclosed in the '926 patent has a rotatable spool upon which seat belt webbing is wound. The webbing has a retracted position in which it is located beside a vehicle seat, and has an extended position in which it extends around a seated vehicle occupant. The spool in the retractor rotates in an unwinding direction as the webbing is withdrawn from the retractor upon movement of the webbing from the retracted position toward the extended position. A rewind spring in the retractor rotates the spool oppositely in a winding direction to retract the webbing back into the retractor, and thus to move the webbing back from the extended position toward the retracted position.
The web-sensitive locking mechanism operates to block rotation of the spool in the unwinding direction upon the occurrence of a vehicle collision. When the vehicle experiences a collision, a vehicle occupant wearing the seat belt webbing moves suddenly against the webbing. This causes the spool to accelerate suddenly in the unwinding direction. The web-sensitive locking mechanism includes a flywheel which lags rotation of the spool when the spool accelerates in the unwinding direction at a predetermined rate which indicates the occurrence of a collision. The web-sensitive locking mechanism blocks rotation of the spool in the unwinding direction in response to such lagging rotation of the flywheel.
When the vehicle occupant withdraws the webbing from the retractor upon movement of the webbing from the retracted position toward the extended position, it is possible for the vehicle occupant to move the webbing quickly enough for the spool to accelerate in the unwinding direction at the predetermined rate. The vehicle occupant can thus move the webbing quickly enough to actuate the web-sensitive locking mechanism before the webbing is moved fully to its extended position. If the web-sensitive locking mechanism is actuated in this manner, the vehicle occupant can allow the rewind spring to retract the webbing back into the retractor until the web-sensitive locking mechanism releases the spool to allow the spool again to rotate in the unwinding direction.